The invention relates to a method and apparatus for detecting an object that is within a heating source""s radiating field, i.e., within the radiating beam of the heating source. More particularly, the invention relates to a method and apparatus for detecting an object in close proximity to a radiant heater, thereby causing the radiant heater to be turned off and an alarm signal to be generated.
Radiant heating appliances typically employ heating elements that consist of quartz tubes surrounding a metal heating element. Electric current is passed through the heating element that increases in temperature and in turn heats the surrounding quartz tube to incandescent temperatures. The heating elements are supported by a structure that serves several purposes. The structure protects the heating elements from coming in contact with objects with a grill. The structure also supports a reflector that is positioned behind, above, and below the elements in order to redirect the thermal radiation from the heating elements outward. Control electronics provide a means to limit the temperature of the elements and to shut the elements down in the event an excessive temperature is sensed.
Radiant energy is the primary source of heating power produced by the appliance. The high temperature of the elements and an unobstructed path from the elements or reflector to the surrounding area allow the appliance to heat objects in its beam. The appliance is typically employed in a home or office to supplement the primary heating system in the building. Unfortunately, there are numerous materials found in these environments that are flammable and present a danger of combustion. Curtains, bed sheets, and paper are typical examples of such materials. There is a possibility that these materials could accidentally come in close proximity to the heating appliance while the unit is powered and radiating. If the materials are close enough to the radiant elements they will be heated by the radiation to a temperature in excess of their combustion temperature and will ignite. The usual safety measures employed in the design of the heating appliance reduce the possibility of ignition. A grill serves to obstruct a material that would otherwise come in contact with the elements. The grill becomes more effective as the ratio of open area to grill work is reduced. The grill cannot be made 100 percent effective however since that would either severely limit or prevent the radiation from escaping from the heater. Therefore the grill is only partially effective as a safety device for the prevention of fire.
Thermal limit or safety switches can be deployed to sense the temperature within the heater and to shut off the flow of power to the elements when an excessive temperature is detected. These devices are typically a bimetal snap acting contact that activates when the body of the switch has reached a limiting threshold temperature. The device has a large thermal mass and requires tens of seconds to reach the threshold temperature where it activates. The switch will not activate in normal operation when radiated energy is escaping the heater. When a sufficient amount of the radiant energy is prevented from escaping the temperature of the switch increases and the heater may eventually be shut down by the switch. This would not occur if either the radiant energy is only partially blocked or insufficient time is allowed for the switch to heat. Unfortunately, it is possible for a small strip of combustible material to come in close proximity with the heater such that the material is heated to its combustion temperature without sufficient radiation being blocked to heat the safety switch. It is also possible for a material with a low combustion temperature to come in close proximity and to heat to its combustion temperature quickly before the thermal safety switch has time to reach its threshold temperature and shut off the radiant power. In both of these cases the thermal safety switch is unable to prevent the combustion of material. The thermal safety switch is only partially effective in preventing accidental combustion.
There are several other inventions that contain devices that sense materials in proximity to the heater. These employ different sensing technologies to detect when materials are dangerously close. Motion detectors have been applied that detect thermal variations from objects moving in their field of view. A motion sensing device is not able to detect a slow moving or stationary target and may not be completely effective at sensing materials that are close enough to combust but remain stationary.
Another technology utilizes a projected beam from an LED and an infrared sensor to detect when the projected light from the LED is reflected from a material in close proximity. This is effective only when the material is present within the projected beam from the LED. An array of LEDS is employed to enlarge the protected area. The array is costly and increases complexity of the heater control system. It is difficult to project light from the LED array in such a way that the entire area illuminated by the heating elements is completely protected as a result of the restrictions imposed on the placement of the emitters and detectors by the heating elements and supporting structures. Materials that are located outside of the area protected by the array but within the radiated energy from the heater can reach combustion temperatures without being sensed. This technology is not completely effective at preventing accidental combustion.
Thus, there is a need in the art for an inexpensive, and more effective technology that can quickly sense when materials are located in close proximity to radiant heaters within their projected beam and shut the power to the heater elements in order to prevent accidental combustion.
The present invention is a method and apparatus for sensing the proximity to a heating source, e.g., a radiant heater, of a foreign material within the heaters beam and shut off the power to the radiant elements to prevent accidental combustion. The invention improves the speed of response and the sensitivity anywhere within the radiant heater beam for detection of objects, including small objects.
To sense when an object is nearby the invention incorporates a thermopile infrared radiation detector. The heaters radiant elements project a beam of infrared radiation during operation. When an object comes in close proximity to the heater, radiation is reflected and absorbed by the object. The thermopile detector is made to be sensitive to the predominant wavelengths of the reflected radiation. The portion of the radiation absorbed by the object causes the object to heat. As the object heats the amount of thermal radiation it emits increases. The thermopile sensor is also made to be sensitive to these emitted wavelengths. Thus the detector is able to receive both reflected and emitted radiation from the object.
The detector is prevented from receiving radiation directly from the radiant heater elements by shielding. The shielding serves both to block direct radiation or radiation reflected from the heater components from being received and to function as an aperture to define the field of view of the detector. The shielding can also serve as a concentrator to collect more energy from the target and focus it onto the detector in order to increase the signal received from foreign materials. The shielding is arranged to reduce the likelihood that direct sunlight could reach the detector.
An electronic control circuit is incorporated that amplifies, filters, and digitizes the signal from the thermopile detector. The temperature of the detector affects the thermopile detector response. A temperature sensor is included within the detector to measure the detector temperature and compensate the detector response. The control circuitry includes amplifiers, filters and analog to digital converters to process the signal from the temperature sensor. The digitized signal information is processed by a microcontroller incorporated in the circuit. The controller executes a method that utilizes the temperature and radiation measurements and decides when an excess of radiation has been detected and if so activates a switching device to stop the flow of electrical power to the radiant heater elements.
An indicator alerts people in the vicinity of the heater that the detector circuit has been activated and that a potentially dangerous situation exists and requires attention. Since the radiant heating elements provide the source of the power that is detected, the alarm signal will disappear when the power to the radiant elements is removed. This requires a period of time as the heater elements cool slowly.
Once the elements are cooled and the alarm signal has disappeared the sensing circuit will not detect the object. To prevent the heater from cycling back on, the present control method holds the heater in the off state. An external input device is included to provide a means to signal the processor that the object has been removed and the heating power can be reapplied. Additionally a method can attempt to generate heating power after a period of time without detecting an object for a predetermined number of trials in order to provide an automatic recovery. In fact, the microcontroller can be programmed with any number of different control methods which utilize the input and output devices described to effect the control of the heater element power.
The method of sensing utilized in the invention has several advantages over the prior art. Since the method employs the radiant power of the heater itself, any object placed within its beam will naturally be illuminated. The detector and shielding is arranged so that radiation can reach the sensor from any location within the beam. This combination provides near 100 percent coverage of the area where there is a danger of accidental combustion. In addition, the sensor is capable of detecting both the reflected and radiated energy from a foreign object placed in close proximity within the heater beam. Radiated energy is produced in increasing amounts as the temperature of the foreign object increases. The sensor reacts very quickly to this increased radiation and can shut the heater off before the object reaches dangerous temperatures close to the combustion point. The sensor therefore can detect a dangerous situation by sensing the dangerous condition of high temperature directly and is more effective than other methods that can only detect the proximity of either a moving or stationary object. The sensing system is simplified by utilizing the radiant heating elements themselves as the source of energy, thereby eliminating the need for any additional sources and their associated cost. The very high power of the heating elements is used to advantage to produce strong signals that are easily distinguished from noise and interference. High signal strength allows the system to readily detect small objects.
The invention therefore describes an effective, and inexpensive method and apparatus that quickly detects small foreign objects located anywhere within the heating beam in close proximity to a radiant heating appliance and shuts power off to the heating elements before the object reaches dangerous temperatures.